1. Field of the Invention
The present invention relates to a correction circuit for a digital quadrature-signal pair in a digital color television receiver where the digital quadrature-signal pair comprises an in-phase signal and a quadrature signal.
2. Description of the Related Art
Analog or digital quadrature-signal pairs are used where two signals are to be transmitted simultaneously by means of a single carrier, e.g., in accordance with conventional color-television standards or by digital quadrature-amplitude modulation, but increasingly also where signals are converted to a different frequency band, e.g., for low-IF conversion of arbitrarily modulated signals, in some single-sideband conversion methods or in the digital processing of radar signals. The correction circuit according to the invention can be used to advantage for angle-modulated signals.
In all these cases, exact processing of the two quadrature components is possible only if the respective frequency components of the analog or digital quadrature-signal pair have precisely the same amplitudes and differ in phase by exactly 90.degree.. In the low-IF method, in which the quadrature-signal pair is formed at the receiving end by analog quadrature mixing, hardly avoidable asymmetries in the two signal paths cause deviations which result in intolerable disturbances.
A remedy for this are correction circuits in which suitable detectors detect errors or interfering components in the respective amplitude and phase of the quadrature-signal pair and derive correction signals therefrom, so that the errors can be eliminated as far as possible. Such a correction circuit is described for analog quadrature-signal pairs in EP-A No. 122 657, corresponding to U.S. Pat. No. 4,633,315, for a low-IF conversion in which an RF television signal is converted to the baseband.
The transition from analog to digital signal processing is very advantageous for such quadrature-signal pairs because a large part of the asymmetries of the two signal paths, such as different aging rates, different influences of temperature, changes in alignment settings, and different internal or external interference- or useful-signal cross-coupling, is eliminated, and because digital technology permits the implementation of complicated filter and processing circuits which can hardly be realized in analog technology. However, the advantages resulting from the use of digital technology can only be achieved if the accuracy of the digital quadrature-signal pair meets more stringent requirements. Complicated correction circuits for this purpose can be implemented to advantage using digital technology. Such a digital correction circuit for phase and amplitude correction is disclosed, for example, in EP-A No. 237 590, corresponding to U.S. Pat. No. 4,799,212. The respective correction signals are added to and subtracted from the respective quadrature component by means of adders and subtracters, the respective correction signal being formed by multiplying one of the quadrature components by the associated correction factor, which is derived from the associated error signal by means of a control circuit.
The main problem in any correction circuit lies in the formation of the error signals. The determination of a deviation is relatively simple if, like in the composite color signal, the RF carrier, which also serves as a picture carrier, is transmitted with a defined amplitude during the horizontal synchronizing pulse in each picture line, and if for the burst signal, the RF carrier is modulated in each picture line with the chrominance subcarrier, whose amplitude and phase are precisely defined during this period. The quadrature-signal pair, which is defined by the burst signal, then represents a vector of constant length rotating at a constant frequency.
The picture-carrier wave during the synchronizing pulse and the chrominance-subcarrier wave during the burst signal can thus be used as a reference during low-IF conversion and color-signal processing, respectively. During low-IF conversion, the phase error, for example, can be determined by multiplying the in-phase signal by the quadrature signal, and the amplitude error by comparing (i.e., taking the difference of) the average amplitude square values of the in-phase and quadrature signals.
During the low-IF conversion of signals not transmitted by means of a carrier or subcarrier, however, this kind of error-signal formation is not possible since the resultant formed by the quadrature-signal pair is constantly modulated and at no time contains a defined reference quantity.